The present invention relates to a charged particle beam apparatus for performing measurement, observation or inspection on a fine pattern formed on a sample, an image forming method using the charged particle beam apparatus, and an image processing apparatus.
In a charged particle beam apparatus to perform measurement, observation or inspection on a fine pattern formed on a sample, a detector having a combination of a scintillator and a photomultiplier tube is employed to detect secondary charged particles which is generated when the sample is irradiated with a charged particle beam. When miniaturization of the pattern as the subject of the measurement, observation or inspection is further advanced, the amount of secondary charged particles which is generated when the sample is irradiated with the charged particle beam is reduced, and the secondary charged particles are discretely incident on the detector. As a result, the detector that has detected such secondary charged particles outputs a pulse-type signal.
As a method of processing the pulse signal outputted from the detector, pulse counting is known. As prior techniques related to the pulse counting, International Patent Publication WO2013/187511, Japanese Unexamined Patent Application Publication No. Hei 08-062274 and Japanese Unexamined Patent Application Publication No. 2011-175811 are known.
International Patent Publication WO2013/187511 discloses “to provide an optical signal detecting circuit, light volume detecting device, and charged particle beam device capable of discriminating, with a simple operation, the signal component of an extremely small amount of light and the signal noise component caused by dark current. A data processor (441) obtains a peak value, which is the maximum voltage value of a pulse, by detecting the pulse from a digital voltage signal that corresponds to an amount of light converted by an amplifier (41) and an A/D converter (42), and stores the occurrence frequency for each found peak value in a frequency storage area (451); a data analyzer (442) compares the occurrence frequencies of the peak values associated with the peak values stored in the frequency storage area (451), sequentially from the smallest value, with a predetermined lower frequency limit, and sets the peak value for which the occurrence frequency is less than or equal to the lower frequency limit as the comparison result, as a pulse determination threshold value; and a threshold value processor (43) outputs a digital voltage signal equal to or greater than the pulse determination threshold value as a detection signal”.
Japanese Unexamined Patent Application Publication No. Hei 08-062274 discloses as follows: “an input is compared with a threshold value Vth in a comparator 2. A threshold-value setting circuit 1 determines the threshold value Vth at any time. The comparator 2 generates a reading signal 3 for an input Vj exceeding the threshold value Vth. The threshold value Vth is changed in correspondence with the magnitude and the frequency of the input signal so that the number of the input signals equal to or greater than the threshold value Vth in the unit time is always contained in the certain range. When the noise level is low, the threshold value Vth becomes low, and the value is measured through the low noise level. Therefore, it is found that the measuring instrument is not defective. When the noise is large, the threshold value Vth is increased, so as to suppress input of noise. Even in this case, it is possible to measure the signal of the partial discharge without omission”.
Japanese Unexamined Patent Application Publication No. 2011-175811 describes a charged particle beam device which “includes a determiner for determining whether an output signal is an output signal in the condition that one charged particle is incident on a detector or the output signal is an output signal in the condition that a plurality of charged particles are incident on the detector; and an operation unit for performing image formation with signal processing based on a pulse count method when it is determined that the output signal is an output signal in the condition that one charged particle is incident on the detector, and performing the image formation with signal processing based on an analog method when it is determined that the output signal is an output signal in the condition that the plurality of charged particles are incident on the detector”.